The f/ratio and the magnification/picture angle are directly related. Think of it like this:
At lower magnifications a given object is imaged by a smaller area of the CCD in less detail, therefore you need less photons. At higher magnifications the same object is being imaged in much greater detail by a larger area of the CCD therefore you need more photons.
An f/6.3 telescope of a given objective diameter provides a lower magnification than an f/13 telescope of the same objective size, therefore it will take less time to image the target. If both telescopes are the same focal length an f/6.3 telescope will have a larger objective than it's f/13 counterpart and will provide a lower magnification, therefore it will again be faster in terms of exposure time.
Slower exposures mean that a typical CCD will suffer from more noise and guidance error (star trailing) will be more pronounced. Faster exposures reduce both of these problems. Faster also means that you can capture more frames for stacking in a given period of time. When imagers talk about fast scopes they generally mean an optical arrangement that's between f/2 and f/6.3 and anything more than f/10 is definitely SLOW when it comes to imaging.
John
Better that old people should die of talk than to have young people die in war.
  